Global warming has had little noticeable impact in Washington, D.C. Politicians in the nation's capital have been reluctant to set limits on the carbon dioxide pollution that is expected to warm the planet by 4 to 7 degrees Fahrenheit during the next century, citing uncertainty about the severity of the threat.

But that uncertainty may have shrunk somewhat with the release last week of two scientific reports suggesting that global warming is not just a hypothetical possibility, but a real phenomenon that has already started transforming especially sensitive parts of the globe.

Overall, the reports say, Earth's climate has warmed by about 1 degree Fahrenheit since 1900. In the Arctic, where a number of processes amplify the warming effects of carbon dioxide, most regions have experienced a temperature rise of 4 to 7 degrees in the last 50 years.

That warmth has reduced the amount of snow that falls every winter, melted away mountain glaciers and shrunk the Arctic Ocean's summer sea ice cover to its smallest extent in millennia, according to satellite measurements. Swaths of Alaskan permafrost are thawing into soggy bogs, and trees are moving northward at the expense of the tundra that rings the Arctic Ocean.

These changes seriously threaten animals such as polar bears, which live and hunt on the sea ice. The bears have already suffered a 15 percent decrease in their number of offspring and a similar decline in weight over the past 25 years. If the Arctic sea ice disappears altogether during the summer months, as some researchers expect it will by the end of the century, polar bears have little chance of survival.

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Things are less serious in the lower 48, where the effects of climate change have been more subtle. In much of the United States, spring arrives about two weeks earlier than it did 50 years ago. Tropical bird species have appeared in Florida and along the Gulf Coast. Species such as Edith's checkerspot, a butterfly native to western North America, have started dying out at the southern reaches of their ranges.

"Responses to climate change are being seen across the U.S.A," said Camille Parmesan, a biologist at the University of Texas in Austin. She is the co-author, with Hector Galbraith of the University of Colorado in Boulder, of "Observed Impacts of Global Climate Change in the U.S." The report was released Tuesday by the Pew Center on Global Climate Change, a non-partisan but not disinterested research organization dedicated to providing sound scientific information about global warming.

Parmesan and Galbraith acknowledge that nothing in the report would strike the average person as particularly alarming. They also allow that some of the past century's warming might have happened even if humans hadn't been pumping carbon dioxide into the atmosphere. But they argue that the changes they describe should be taken as a "very clear signal" that climate change will have significant effects in coming decades.

"The canaries in the coalmine are squawking, and we should absolutely take that seriously," Galbraith said.

The Bush administration has argued that not enough is known about climate change to justify major efforts at forestalling or preventing future warming.

The Arctic report, released Monday, was commissioned by the Arctic Council, an international commission of eight countries, including the United States, and six indigenous groups. It was written by a team of 300 scientists.

"The report will be a valuable contribution to the literature on potential regional impacts of climate change, and the United States government will take its findings into account as it continues to review the science," State Department spokesman Richard Boucher said in a statement released Tuesday.

The United States faces a potential showdown with other members of the Arctic Council on Nov. 24, when representatives of the organization's members are scheduled to meet in Iceland to consider climate change policy recommendations.

The atmospheric concentration of carbon dioxide has risen from 280 parts per million in 1800 to 380 parts per million today due to the combustion of fossil fuels. Carbon dioxide causes warming because it heats up more when exposed to sunlight compared to other atmospheric gases.

Scientists have always expected the Arctic to respond earlier and more intensely than other regions to the buildup of carbon dioxide in the atmosphere, thanks to several phenomena that make the far north especially sensitive to climate perturbations. When warmer temperatures melt snow, for example, the bare ground that is exposed absorbs more heat than the white surface did, causing yet more warming. A similar thing happens when sea ice melts, exposing open water.

In the past three Septembers the Arctic sea ice has melted back 12 percent to 15 percent beyond its normal minimum extent.

"It almost suggests that maybe we're about to reach a threshold beyond which the sea ice may not be able to recover," said Mark Serreze of the National Snow and Ice Data Center in Boulder, Colo.

Ice in the interior of the Arctic pack normally remains frozen from year to year, growing thicker with each season. But the recent increase in melting has eaten into much of that multi-year ice. So while the Arctic Ocean still freezes over each winter, more of the solid cover now consists of thin single-year ice that melts every spring.

The Arctic is also particularly sensitive to warming because its plants and soil hold less water than more temperate environments. That means more energy reaching the ground is dedicated to heating the surface instead of evaporating water.

The atmosphere is thinner in the Arctic than it is farther south, which also intensifies warming. And while temperate zones shed some of their extra heat by shipping it north in ocean currents and meteorological fronts, the Arctic is the end of the line in that respect.

A small minority of scientists remains unconvinced that increasing atmospheric carbon dioxide can be held responsible for the recent warming, arguing that natural variability explains most if not all of the trend.

"It's very complicated and I believe people who claim they understand ... are just overestimating drastically their ability to do science," Chylek said.

Scientists aren't the only ones who have noticed the Arctic warming trend. Inuit hunters in Canada and Saami reindeer herders in Finland have detected shifts in the migratory behavior of animals. In some cases, people whose elders taught them decades ago how to forecast storms from wind patterns and cloud formations have lost their predictive abilities to new weather patterns.

"One of the unique things about Arctic communities is how much they're tied to the land, and that's why this is such a big deal for them," said Harvard University geographer Shari Fox Gearheard.

Farther south, where the changes have been far less extreme and most people live far removed from the subtleties of their climate, a warmer world remains a hypothetical realm of scientists and environmentalists. But the latest reports suggest that in some of the world's more populated places, astute observers may soon begin to noticing that the climate is changing.